University of California Press
Westem Gulls (Larus occidentalis) at Bodega Bay, California drop shelled prey items to break them. I presented Washington clams (Saxidomus nuttalli) of known weight to free-flying gulls to investigate factors affecting shell-dropping behavior. All adult gulls dropped clams, whereas only 55% of immature gulls did so. The other 45% of immature gulls that were given clams pecked at them on the ground instead. Gulls dropped clams on both hard and soft substrates. Flight distance and kleptoparasitism seemed important in influencing drop location. Adult Western Gulls dropped heavy clams from lower heights than they dropped light clams. Heavy clams, however, break less easily than light clams when dropped from the same height. Energetic constraints and/or kleptoparasitism could explain this apparent contradiction.
Copyright 1982 by the American Ornithologists' Union